auto merge of #6260 : alexcrichton/rust/issue-3466-no-swap, r=pcwalton

There may be a more efficient implementation of `core::util::swap_ptr`. The issue mentioned using `move_val_init`, but I couldn't figure out what that did, so I just used `copy_memory` a few times instead.

I'm not exactly the best at reading LLVM generated by rust, but this does appear to be optimized away just as expected (when possible).

6 files changed, 138 insertions, 104 deletions

diff --git a/src/libcore/cell.rs b/src/libcore/cell.rs
index c7f9e377571..18e75fb1aa9 100644
--- a/src/libcore/cell.rs
+++ b/src/libcore/cell.rs
@@ -12,6 +12,7 @@
 
 use cast::transmute_mut;
 use prelude::*;
+use util::replace;
 
 /*
 A dynamic, mutable location.
@@ -48,9 +49,7 @@ pub impl<T> Cell<T> {
             fail!(~"attempt to take an empty cell");
         }
 
-        let mut value = None;
-        value <-> self.value;
-        value.unwrap()
+        replace(&mut self.value, None).unwrap()
     }
 
     /// Returns the value, failing if the cell is full.
diff --git a/src/libcore/comm.rs b/src/libcore/comm.rs
index 7eaa8535493..140eb41fdf3 100644
--- a/src/libcore/comm.rs
+++ b/src/libcore/comm.rs
@@ -21,6 +21,7 @@ use uint;
 use unstable;
 use vec;
 use unstable::Exclusive;
+use util::replace;
 
 use pipes::{recv, try_recv, wait_many, peek, PacketHeader};
 
@@ -149,9 +150,8 @@ impl<T: Owned> GenericChan<T> for Chan<T> {
     #[inline(always)]
     fn send(&self, x: T) {
         unsafe {
-            let mut endp = None;
             let mut self_endp = transmute_mut(&self.endp);
-            endp <-> *self_endp;
+            let endp = replace(self_endp, None);
             *self_endp = Some(streamp::client::data(endp.unwrap(), x))
         }
     }
@@ -161,9 +161,8 @@ impl<T: Owned> GenericSmartChan<T> for Chan<T> {
     #[inline(always)]
     fn try_send(&self, x: T) -> bool {
         unsafe {
-            let mut endp = None;
             let mut self_endp = transmute_mut(&self.endp);
-            endp <-> *self_endp;
+            let endp = replace(self_endp, None);
             match streamp::client::try_data(endp.unwrap(), x) {
                 Some(next) => {
                     *self_endp = Some(next);
@@ -179,9 +178,8 @@ impl<T: Owned> GenericPort<T> for Port<T> {
     #[inline(always)]
     fn recv(&self) -> T {
         unsafe {
-            let mut endp = None;
             let mut self_endp = transmute_mut(&self.endp);
-            endp <-> *self_endp;
+            let endp = replace(self_endp, None);
             let streamp::data(x, endp) = recv(endp.unwrap());
             *self_endp = Some(endp);
             x
@@ -191,9 +189,8 @@ impl<T: Owned> GenericPort<T> for Port<T> {
     #[inline(always)]
     fn try_recv(&self) -> Option<T> {
         unsafe {
-            let mut endp = None;
             let mut self_endp = transmute_mut(&self.endp);
-            endp <-> *self_endp;
+            let endp = replace(self_endp, None);
             match try_recv(endp.unwrap()) {
                 Some(streamp::data(x, endp)) => {
                     *self_endp = Some(endp);
@@ -209,14 +206,13 @@ impl<T: Owned> Peekable<T> for Port<T> {
     #[inline(always)]
     fn peek(&self) -> bool {
         unsafe {
-            let mut endp = None;
             let mut self_endp = transmute_mut(&self.endp);
-            endp <-> *self_endp;
+            let mut endp = replace(self_endp, None);
             let peek = match endp {
                 Some(ref mut endp) => peek(endp),
                 None => fail!(~"peeking empty stream")
             };
-            *self_endp <-> endp;
+            *self_endp = endp;
             peek
         }
     }
@@ -267,8 +263,7 @@ impl<T:Owned> GenericPort<T> for PortSet<T> {
             let mut result = None;
             // we have to swap the ports array so we aren't borrowing
             // aliasable mutable memory.
-            let mut ports = ~[];
-            ports <-> *self_ports;
+            let mut ports = replace(self_ports, ~[]);
             while result.is_none() && ports.len() > 0 {
                 let i = wait_many(ports);
                 match ports[i].try_recv() {
@@ -281,7 +276,7 @@ impl<T:Owned> GenericPort<T> for PortSet<T> {
                     }
                 }
             }
-            ports <-> *self_ports;
+            *self_ports = ports;
             result
         }
     }
@@ -320,8 +315,7 @@ impl<T: Owned> GenericChan<T> for SharedChan<T> {
     fn send(&self, x: T) {
         let mut xx = Some(x);
         do self.ch.with_imm |chan| {
-            let mut x = None;
-            x <-> xx;
+            let x = replace(&mut xx, None);
             chan.send(x.unwrap())
         }
     }
@@ -331,8 +325,7 @@ impl<T: Owned> GenericSmartChan<T> for SharedChan<T> {
     fn try_send(&self, x: T) -> bool {
         let mut xx = Some(x);
         do self.ch.with_imm |chan| {
-            let mut x = None;
-            x <-> xx;
+            let x = replace(&mut xx, None);
             chan.try_send(x.unwrap())
         }
     }
diff --git a/src/libcore/hashmap.rs b/src/libcore/hashmap.rs
index b5ae07208fc..590d4ab3bcb 100644
--- a/src/libcore/hashmap.rs
+++ b/src/libcore/hashmap.rs
@@ -176,16 +176,13 @@ priv impl<K:Hash + Eq,V> HashMap<K, V> {
     /// Expands the capacity of the array and re-insert each of the
     /// existing buckets.
     fn resize(&mut self, new_capacity: uint) {
-        let old_capacity = self.buckets.len();
         self.resize_at = resize_at(new_capacity);
 
-        let mut old_buckets = vec::from_fn(new_capacity, |_| None);
-        self.buckets <-> old_buckets;
+        let old_buckets = replace(&mut self.buckets,
+                                  vec::from_fn(new_capacity, |_| None));
 
         self.size = 0;
-        for uint::range(0, old_capacity) |i| {
-            let mut bucket = None;
-            bucket <-> old_buckets[i];
+        do vec::consume(old_buckets) |_, bucket| {
             self.insert_opt_bucket(bucket);
         }
     }
@@ -265,13 +262,11 @@ priv impl<K:Hash + Eq,V> HashMap<K, V> {
         };
 
         let len_buckets = self.buckets.len();
-        let mut bucket = None;
-        self.buckets[idx] <-> bucket;
+        let bucket = replace(&mut self.buckets[idx], None);
 
         let value = match bucket {
             None => None,
-            Some(bucket) => {
-                let Bucket{value: value, _} = bucket;
+            Some(Bucket{value, _}) => {
                 Some(value)
             },
         };
@@ -281,8 +276,7 @@ priv impl<K:Hash + Eq,V> HashMap<K, V> {
         let size = self.size - 1;
         idx = self.next_bucket(idx, len_buckets);
         while self.buckets[idx].is_some() {
-            let mut bucket = None;
-            bucket <-> self.buckets[idx];
+            let bucket = replace(&mut self.buckets[idx], None);
             self.insert_opt_bucket(bucket);
             idx = self.next_bucket(idx, len_buckets);
         }
@@ -613,15 +607,13 @@ pub impl<K: Hash + Eq, V> HashMap<K, V> {
     }
 
     fn consume(&mut self, f: &fn(K, V)) {
-        let mut buckets = ~[];
-        self.buckets <-> buckets;
+        let buckets = replace(&mut self.buckets, ~[]);
         self.size = 0;
 
         do vec::consume(buckets) |_, bucket| {
             match bucket {
                 None => {},
-                Some(bucket) => {
-                    let Bucket{key: key, value: value, _} = bucket;
+                Some(Bucket{key, value, _}) => {
                     f(key, value)
                 }
             }
diff --git a/src/libcore/pipes.rs b/src/libcore/pipes.rs
index 8301254fbdd..fe9c78198bb 100644
--- a/src/libcore/pipes.rs
+++ b/src/libcore/pipes.rs
@@ -93,6 +93,7 @@ use unstable::intrinsics;
 use ptr;
 use task;
 use vec;
+use util::replace;
 
 static SPIN_COUNT: uint = 0;
 
@@ -428,8 +429,7 @@ fn try_recv_<T:Owned>(p: &mut Packet<T>) -> Option<T> {
     // optimistic path
     match p.header.state {
       Full => {
-        let mut payload = None;
-        payload <-> p.payload;
+        let payload = replace(&mut p.payload, None);
         p.header.state = Empty;
         return Some(payload.unwrap())
       },
@@ -480,8 +480,7 @@ fn try_recv_<T:Owned>(p: &mut Packet<T>) -> Option<T> {
             fail!(~"blocking on already blocked packet")
           },
           Full => {
-            let mut payload = None;
-            payload <-> p.payload;
+            let payload = replace(&mut p.payload, None);
             let old_task = swap_task(&mut p.header.blocked_task, ptr::null());
             if !old_task.is_null() {
                 unsafe {
@@ -675,8 +674,7 @@ impl<T:Owned,Tbuffer:Owned> Drop for SendPacketBuffered<T,Tbuffer> {
         unsafe {
             let this: &mut SendPacketBuffered<T,Tbuffer> = transmute(self);
             if this.p != None {
-                let mut p = None;
-                p <-> this.p;
+                let p = replace(&mut this.p, None);
                 sender_terminate(p.unwrap())
             }
         }
@@ -695,9 +693,7 @@ pub fn SendPacketBuffered<T,Tbuffer>(p: *mut Packet<T>)
 
 pub impl<T,Tbuffer> SendPacketBuffered<T,Tbuffer> {
     fn unwrap(&mut self) -> *mut Packet<T> {
-        let mut p = None;
-        p <-> self.p;
-        p.unwrap()
+        replace(&mut self.p, None).unwrap()
     }
 
     fn header(&mut self) -> *mut PacketHeader {
@@ -713,9 +709,7 @@ pub impl<T,Tbuffer> SendPacketBuffered<T,Tbuffer> {
 
     fn reuse_buffer(&mut self) -> BufferResource<Tbuffer> {
         //error!("send reuse_buffer");
-        let mut tmp = None;
-        tmp <-> self.buffer;
-        tmp.unwrap()
+        replace(&mut self.buffer, None).unwrap()
     }
 }
 
@@ -738,8 +732,7 @@ impl<T:Owned,Tbuffer:Owned> Drop for RecvPacketBuffered<T,Tbuffer> {
         unsafe {
             let this: &mut RecvPacketBuffered<T,Tbuffer> = transmute(self);
             if this.p != None {
-                let mut p = None;
-                p <-> this.p;
+                let p = replace(&mut this.p, None);
                 receiver_terminate(p.unwrap())
             }
         }
@@ -748,15 +741,11 @@ impl<T:Owned,Tbuffer:Owned> Drop for RecvPacketBuffered<T,Tbuffer> {
 
 pub impl<T:Owned,Tbuffer:Owned> RecvPacketBuffered<T, Tbuffer> {
     fn unwrap(&mut self) -> *mut Packet<T> {
-        let mut p = None;
-        p <-> self.p;
-        p.unwrap()
+        replace(&mut self.p, None).unwrap()
     }
 
     fn reuse_buffer(&mut self) -> BufferResource<Tbuffer> {
-        let mut tmp = None;
-        tmp <-> self.buffer;
-        tmp.unwrap()
+        replace(&mut self.buffer, None).unwrap()
     }
 }
 
diff --git a/src/libcore/util.rs b/src/libcore/util.rs
index 43616ebfd30..db9a17cf97f 100644
--- a/src/libcore/util.rs
+++ b/src/libcore/util.rs
@@ -15,6 +15,7 @@ Miscellaneous helpers for common patterns.
 */
 
 use prelude::*;
+use unstable::intrinsics;
 
 /// The identity function.
 #[inline(always)]
@@ -34,12 +35,12 @@ pub fn ignore<T>(_x: T) { }
 #[inline(always)]
 pub fn with<T,R>(
     ptr: @mut T,
-    mut value: T,
+    value: T,
     op: &fn() -> R) -> R
 {
-    value <-> *ptr;
+    let prev = replace(ptr, value);
     let result = op();
-    *ptr = value;
+    *ptr = prev;
     return result;
 }
 
@@ -49,7 +50,65 @@ pub fn with<T,R>(
  */
 #[inline(always)]
 pub fn swap<T>(x: &mut T, y: &mut T) {
-    *x <-> *y;
+    unsafe {
+        swap_ptr(ptr::to_mut_unsafe_ptr(x), ptr::to_mut_unsafe_ptr(y));
+    }
+}
+
+/**
+ * Swap the values at two mutable locations of the same type, without
+ * deinitialising or copying either one.
+ */
+#[inline]
+#[cfg(not(stage0))]
+pub unsafe fn swap_ptr<T>(x: *mut T, y: *mut T) {
+    if x == y { return }
+
+    // Give ourselves some scratch space to work with
+    let mut tmp: T = intrinsics::uninit();
+    let t = ptr::to_mut_unsafe_ptr(&mut tmp);
+
+    // Perform the swap
+    ptr::copy_memory(t, x, 1);
+    ptr::copy_memory(x, y, 1);
+    ptr::copy_memory(y, t, 1);
+
+    // y and t now point to the same thing, but we need to completely forget t
+    // because it's no longer relevant.
+    cast::forget(tmp);
+}
+
+/**
+ * Swap the values at two mutable locations of the same type, without
+ * deinitialising or copying either one.
+ */
+#[inline]
+#[cfg(stage0)]
+pub unsafe fn swap_ptr<T>(x: *mut T, y: *mut T) {
+    if x == y { return }
+
+    // Give ourselves some scratch space to work with
+    let mut tmp: T = intrinsics::init();
+    let t = ptr::to_mut_unsafe_ptr(&mut tmp);
+
+    // Perform the swap
+    ptr::copy_memory(t, x, 1);
+    ptr::copy_memory(x, y, 1);
+    ptr::copy_memory(y, t, 1);
+
+    // y and t now point to the same thing, but we need to completely forget t
+    // because it's no longer relevant.
+    cast::forget(tmp);
+}
+
+/**
+ * Replace the value at a mutable location with a new one, returning the old
+ * value, without deinitialising or copying either one.
+ */
+#[inline(always)]
+pub fn replace<T>(dest: &mut T, mut src: T) -> T {
+    swap(dest, &mut src);
+    src
 }
 
 /**
@@ -57,10 +116,9 @@ pub fn swap<T>(x: &mut T, y: &mut T) {
  * value, without deinitialising or copying either one.
  */
 #[inline(always)]
-pub fn replace<T>(dest: &mut T, src: T) -> T {
-    let mut tmp = src;
-    swap(dest, &mut tmp);
-    tmp
+pub unsafe fn replace_ptr<T>(dest: *mut T, mut src: T) -> T {
+    swap_ptr(dest, ptr::to_mut_unsafe_ptr(&mut src));
+    src
 }
 
 /// A non-copyable dummy type.
diff --git a/src/libcore/vec.rs b/src/libcore/vec.rs
index 7eba2cbf0cc..77314b173d9 100644
--- a/src/libcore/vec.rs
+++ b/src/libcore/vec.rs
@@ -29,6 +29,7 @@ use sys;
 use uint;
 use unstable::intrinsics;
 use vec;
+use util;
 
 #[cfg(not(test))] use cmp::Equiv;
 
@@ -470,7 +471,7 @@ pub fn shift<T>(v: &mut ~[T]) -> T {
         let next_ln = v.len() - 1;
 
         // Save the last element. We're going to overwrite its position
-        let mut work_elt = v.pop();
+        let work_elt = v.pop();
         // We still should have room to work where what last element was
         assert!(capacity(v) >= ln);
         // Pretend like we have the original length so we can use
@@ -501,16 +502,14 @@ pub fn shift<T>(v: &mut ~[T]) -> T {
         // Swap out the element we want from the end
         let vp = raw::to_mut_ptr(*v);
         let vp = ptr::mut_offset(vp, next_ln - 1);
-        *vp <-> work_elt;
 
-        work_elt
+        util::replace_ptr(vp, work_elt)
     }
 }
 
 /// Prepend an element to the vector
 pub fn unshift<T>(v: &mut ~[T], x: T) {
-    let mut vv = ~[x];
-    *v <-> vv;
+    let vv = util::replace(v, ~[x]);
     v.push_all_move(vv);
 }
 
@@ -523,7 +522,7 @@ pub fn insert<T>(v: &mut ~[T], i: uint, x: T) {
     v.push(x);
     let mut j = len;
     while j > i {
-        v[j] <-> v[j - 1];
+        swap(*v, j, j - 1);
         j -= 1;
     }
 }
@@ -536,7 +535,7 @@ pub fn remove<T>(v: &mut ~[T], i: uint) -> T {
 
     let mut j = i;
     while j < len - 1 {
-        v[j] <-> v[j + 1];
+        swap(*v, j, j + 1);
         j += 1;
     }
     v.pop()
@@ -550,10 +549,9 @@ pub fn consume<T>(mut v: ~[T], f: &fn(uint, v: T)) {
                 // holes we create in the vector. That ensures that, if the
                 // iterator fails then we won't try to clean up the consumed
                 // elements during unwinding
-                let mut x = intrinsics::init();
+                let x = intrinsics::init();
                 let p = ptr::mut_offset(p, i);
-                x <-> *p;
-                f(i, x);
+                f(i, util::replace_ptr(p, x));
             }
         }
 
@@ -572,10 +570,9 @@ pub fn consume_reverse<T>(mut v: ~[T], f: &fn(uint, v: T)) {
                 // holes we create in the vector. That ensures that, if the
                 // iterator fails then we won't try to clean up the consumed
                 // elements during unwinding
-                let mut x = intrinsics::init();
+                let x = intrinsics::init();
                 let p = ptr::mut_offset(p, i);
-                x <-> *p;
-                f(i, x);
+                f(i, util::replace_ptr(p, x));
             }
         }
 
@@ -592,8 +589,7 @@ pub fn pop<T>(v: &mut ~[T]) -> T {
     }
     let valptr = ptr::to_mut_unsafe_ptr(&mut v[ln - 1u]);
     unsafe {
-        let mut val = intrinsics::uninit();
-        val <-> *valptr;
+        let val = util::replace_ptr(valptr, intrinsics::uninit());
         raw::set_len(v, ln - 1u);
         val
     }
@@ -607,8 +603,7 @@ pub fn pop<T>(v: &mut ~[T]) -> T {
     }
     let valptr = ptr::to_mut_unsafe_ptr(&mut v[ln - 1u]);
     unsafe {
-        let mut val = intrinsics::init();
-        val <-> *valptr;
+        let val = util::replace_ptr(valptr, intrinsics::init());
         raw::set_len(v, ln - 1u);
         val
     }
@@ -626,7 +621,7 @@ pub fn swap_remove<T>(v: &mut ~[T], index: uint) -> T {
         fail!(fmt!("vec::swap_remove - index %u >= length %u", index, ln));
     }
     if index < ln - 1 {
-        v[index] <-> v[ln - 1];
+        swap(*v, index, ln - 1);
     }
     v.pop()
 }
@@ -682,8 +677,8 @@ pub fn push_all_move<T>(v: &mut ~[T], mut rhs: ~[T]) {
     unsafe {
         do as_mut_buf(rhs) |p, len| {
             for uint::range(0, len) |i| {
-                let mut x = intrinsics::uninit();
-                x <-> *ptr::mut_offset(p, i);
+                let x = util::replace_ptr(ptr::mut_offset(p, i),
+                                          intrinsics::uninit());
                 push(&mut *v, x);
             }
         }
@@ -699,8 +694,8 @@ pub fn push_all_move<T>(v: &mut ~[T], mut rhs: ~[T]) {
     unsafe {
         do as_mut_buf(rhs) |p, len| {
             for uint::range(0, len) |i| {
-                let mut x = intrinsics::init();
-                x <-> *ptr::mut_offset(p, i);
+                let x = util::replace_ptr(ptr::mut_offset(p, i),
+                                          intrinsics::init());
                 push(&mut *v, x);
             }
         }
@@ -716,8 +711,7 @@ pub fn truncate<T>(v: &mut ~[T], newlen: uint) {
         unsafe {
             // This loop is optimized out for non-drop types.
             for uint::range(newlen, oldlen) |i| {
-                let mut dropped = intrinsics::uninit();
-                dropped <-> *ptr::mut_offset(p, i);
+                util::replace_ptr(ptr::mut_offset(p, i), intrinsics::uninit());
             }
         }
     }
@@ -732,8 +726,7 @@ pub fn truncate<T>(v: &mut ~[T], newlen: uint) {
         unsafe {
             // This loop is optimized out for non-drop types.
             for uint::range(newlen, oldlen) |i| {
-                let mut dropped = intrinsics::init();
-                dropped <-> *ptr::mut_offset(p, i);
+                util::replace_ptr(ptr::mut_offset(p, i), intrinsics::init());
             }
         }
     }
@@ -758,14 +751,14 @@ pub fn dedup<T:Eq>(v: &mut ~[T]) {
                 // last_written < next_to_read < ln
                 if *ptr::mut_offset(p, next_to_read) ==
                     *ptr::mut_offset(p, last_written) {
-                    let mut dropped = intrinsics::uninit();
-                    dropped <-> *ptr::mut_offset(p, next_to_read);
+                    util::replace_ptr(ptr::mut_offset(p, next_to_read),
+                                      intrinsics::uninit());
                 } else {
                     last_written += 1;
                     // last_written <= next_to_read < ln
                     if next_to_read != last_written {
-                        *ptr::mut_offset(p, last_written) <->
-                            *ptr::mut_offset(p, next_to_read);
+                        util::swap_ptr(ptr::mut_offset(p, last_written),
+                                       ptr::mut_offset(p, next_to_read));
                     }
                 }
                 // last_written <= next_to_read < ln
@@ -796,14 +789,14 @@ pub fn dedup<T:Eq>(v: &mut ~[T]) {
                 // last_written < next_to_read < ln
                 if *ptr::mut_offset(p, next_to_read) ==
                     *ptr::mut_offset(p, last_written) {
-                    let mut dropped = intrinsics::init();
-                    dropped <-> *ptr::mut_offset(p, next_to_read);
+                    util::replace_ptr(ptr::mut_offset(p, next_to_read),
+                                      intrinsics::init());
                 } else {
                     last_written += 1;
                     // last_written <= next_to_read < ln
                     if next_to_read != last_written {
-                        *ptr::mut_offset(p, last_written) <->
-                            *ptr::mut_offset(p, next_to_read);
+                        util::swap_ptr(ptr::mut_offset(p, last_written),
+                                       ptr::mut_offset(p, next_to_read));
                     }
                 }
                 // last_written <= next_to_read < ln
@@ -1028,7 +1021,7 @@ pub fn retain<T>(v: &mut ~[T], f: &fn(t: &T) -> bool) {
         if !f(&v[i]) {
             deleted += 1;
         } else if deleted > 0 {
-            v[i - deleted] <-> v[i];
+            swap(*v, i - deleted, i);
         }
     }
 
@@ -1429,15 +1422,25 @@ pub fn zip<T, U>(mut v: ~[T], mut u: ~[U]) -> ~[(T, U)] {
  * * a - The index of the first element
  * * b - The index of the second element
  */
+#[inline(always)]
 pub fn swap<T>(v: &mut [T], a: uint, b: uint) {
-    v[a] <-> v[b];
+    unsafe {
+        // Can't take two mutable loans from one vector, so instead just cast
+        // them to their raw pointers to do the swap
+        let pa: *mut T = ptr::to_mut_unsafe_ptr(&mut v[a]);
+        let pb: *mut T = ptr::to_mut_unsafe_ptr(&mut v[b]);
+        util::swap_ptr(pa, pb);
+    }
 }
 
 /// Reverse the order of elements in a vector, in place
 pub fn reverse<T>(v: &mut [T]) {
     let mut i: uint = 0;
     let ln = len::<T>(v);
-    while i < ln / 2 { v[i] <-> v[ln - i - 1]; i += 1; }
+    while i < ln / 2 {
+        swap(v, i, ln - i - 1);
+        i += 1;
+    }
 }
 
 /// Returns a vector with the order of elements reversed
@@ -2476,6 +2479,7 @@ pub mod raw {
     use sys;
     use unstable::intrinsics;
     use vec::{UnboxedVecRepr, as_const_buf, as_mut_buf, len, with_capacity};
+    use util;
 
     /// The internal representation of a (boxed) vector
     pub struct VecRepr {
@@ -2573,8 +2577,7 @@ pub mod raw {
     pub unsafe fn init_elem<T>(v: &mut [T], i: uint, val: T) {
         let mut box = Some(val);
         do as_mut_buf(v) |p, _len| {
-            let mut box2 = None;
-            box2 <-> box;
+            let box2 = util::replace(&mut box, None);
             intrinsics::move_val_init(&mut(*ptr::mut_offset(p, i)),
                                       box2.unwrap());
         }